The present invention relates generally to films, and more particularly to the fabrication of porous films. The porous films may be used in a variety of applications ranging, for example, from sensors, to catalysts, to porous media, to fuel cells.
Fuel cells use an electrochemical energy conversion of fuel (including but not limited to hydrogen, propane, methane, carbon monoxide, and the like) and oxidant(s) into electricity and heat. It is anticipated that fuel cells may be able to replace primary and secondary batteries as a portable power supply. In fuel cells, the fuel (usually containing a source of hydrogen) is oxidized typically with a source of oxygen to produce (primarily) water and carbon dioxide. The oxidation reaction at the anode, which liberates electrons, in combination with the reduction reaction at the cathode, which consumes electrons, results in a useful electrical voltage and current through the load.
Fuel cells provide a direct current that may be used to power motors, lights, electrical appliances, etc. A solid oxide fuel cell (SOFC) is one type of fuel cell that may be useful in portable or non-portable applications. Unfortunately, the material properties, which are generally desirable in order to obtain high performing fuel cell devices, also make fabrication of those fuel cell devices a significant challenge.
A thin film approach for the fabrication of SOFCs is relatively new and has many challenges. There are many different techniques for deposition of thin films. Some of them, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), etc., have many advantages, but deposition of porous films (with techniques such as these) for use as, for example, SOFC electrodes, may be a significant challenge. This may be due in part to the fact that these techniques are normally used for deposition of dense films.
Porous films may be deposited (for example, by PVD) by increasing the pressure in a deposition chamber, decreasing the substrate temperature, or increasing the source to substrate distance. However, these techniques generally do not allow deposition of highly porous films. The porosity achievable by these deposition techniques is generally below about 5%. Another technique for depositing a porous film with one-dimensional through channels requires the deposition of two components. The through channels are created when one of the components is selectively removed by etching.